The present invention is related to surface pumping units for deep wells, such as oil wells, and more specifically to hydraulically operated surface pumping units for lifting and lowering pump rods of cylinder pumps in deep wells.
When deep wells, such as oil wells, have insufficient reservoir pressure to drive the formation fluid to the surface of the ground, the wells must be pumped. A variety of pumps are in common use for pumping reservoir fluid to the surface, including down hole turbine pumps, gas lift pumps, vacuum pumps and the like, depending to a large extent on depth, volume flow rates, and other economic as well as aesthetic considerations. However, the most common type of pump used in the past and still used at the present time is a conventional reciprocating piston cylinder or barrel pump. In this type of pump, a cylinder with a piston therein is positioned down hole below the fluid level in the well. Reciprocal vertical movement of the piston up and down in the cylinder, in combination with appropriate check valves, pumps the fluid in the well to the surface of the ground. When using a down hole cylinder or barrel pump therefore, it is necessary to provide a drive mechanism for reciprocating the piston up and down in the pump cylinder. In the conventional manner of driving such pumps, a pump rod is extended downwardly in the well casing from the surface of the ground with the lower end of the pump rod string attached to the piston in the pump cylinder. A drive apparatus capable of moving the pump rod up and down in reciprocal motion is then attached to the pump rod at the well head at the surface of the ground. Such drive apparatus are called surface pumping units.
The most commonly used surface pumping units are in the form of counterbalance cross beam pumps which utilize a large horizontal cross beam pivotally mounted on a frame. The cross beam is driven to rock in an oscillating upstroke and down stroke motion in a vertical plane. The surface end of the pump rod string is attached to one end of the cross beam so that the oscillating motion of the cross beam pulls the pump rod up and lets it down in reciprocal motion. In conventional cross beam surface pumping units the cross beam is usually oscillated by a motor driven crank wheel with a connecting rod extending between an eccentric connection to the crank wheel and the cross beam. The motor or engine used to drive the pump is commonly connected to the crank wheel with gears, chains, belts, or the like. These conventional cross beam surface pumping units are commonly called horse head pumps due to the appearance of the cable mount and guideplate on the end of the cross beam. In order to offset the weight of the pump rod string hanging from one end of the cross beam, the surface pumping units are counterbalanced for smoother operation by hanging counter weights on the opposite end of the cross beam or by attaching eccentric weights on the crank wheel or both.
Such conventional pumps have in the past and still are widely used in the industry and generally perform well. However, there are a number of problems associated with the use of such surface pumping units that have not heretofore been solved. For example, the gear boxes, belts, chains or pulleys used to drive the surface pumping units are in constant need of maintenance and repair and can become particularly troublesome in adverse climatic conditions, such as extreme cold or extreme heat. Counterbalancing has also always been a difficult precedure on such surface pumping units. Installation of a rod dynamometer is usually required, and it is very difficult to avoid hammering the piston of down hole well pump on the bottom of the cylinder during such counterbalancing and initial set-up operations. These problems have become more significant in deep well applications, such as in oil wells 10,000 to 15,000 feet deep. In wells of such depth, the pump rods are very long and heavy, and elastic yield and stretch in the pump rod between the surface and the bottom make it very difficult if not impossible to calculate or predict accurately the relative position of the piston in the cylinder in the down hole pump at any time during the reciprocating phases of the pump cycle.
Other problems associated with the use of conventional surface pumping units include little or no flexibility for varying the up and down stroke lengths or speeds. Also, the desired pumping perameters vary significantly among different formation characteristics of the oil reservoirs. For example, some reservoirs are conducive to very high volume oil flow so fast stroke cycles can be used. Others are relatively impermeable and oil flow is slow pumping so slower pumping and maximum open barrel dwell time is required. In reservoirs where salt water or other undesirable fluids are close to the oil bearing strata, it is often necessary to pump in long slow strokes to avoid drawing salt water into the well. On the other hand where significant amounts of salt water in the produced oil cannot be avoided, it is often desirable to pump the well at a higher rate to produce sufficient fluid to meet quotas allowed for the well. Since conventional surface pumping units have the disadvantage of operating at one unitary speed in both the up and down strokes, such varying conditions are difficult to accomodate.
A relatively recent development in surface pumping units is the use of hydraulic cylinders mounted directly over the well head and connected to the pump rod for pulling the pump rod up and allowing it to fall down again. Such "pedestal" surface pumping units have advantages, such as eliminating large equipment and being able to operate at different varying speeds; however, they have not been effective to solve all the problems mentioned above and are particularly ineffective in extremely deep well application due to the excessive weight of the pump rod strings in such deep well application.